The present invention relates to a method and device for the automatic measurement of the evaporation time of a barium getter.
It is known that evaporable getter devices that are mounted within an evacuated electron tube, generally near to the glass wall, have to be evaporated before use. The evaporation takes place when, generally, the tube has been evacuated and tippedoff. Evaporation of the evaporable getter device takes place by heating the device to a temperature such that the barium contained therein is freed, in the form of vapours, which then deposits in the form of a thin film on surfaces within the tube.
It is also well known that this heating generally takes place by induction heating at high frequency by at least one R.F. coil positioned outside the glass wall at as small a distance as possible from the getter device. U.S. Pat. No. 4,302,063 and European Patent Application No. 0,321,042 describe methods, and relative apparatus, suitable for improving electromagnetic coupling between the coil and getter device and to determine the optimum position of the coil. In this way uncertainties in coil positioning relative to the getter are minimized thus obtaining minimum energy dispersion and maximum heat transfer to the getter device.
Using the apparatus described in the above noted publications it is possible to make this step of production of the vacuum tube automatic with subsequent reductions in cost and increased output, especially in cases of mass production as for example in the field of colour television picture tube.
However, it has not been possible, up to now, to take advantage of all the benefits offered by automating these operations because of the difficulties met with in determining the so called "start time" of evaporation of each individual getter device. By the term "start time" or time when barium starts to evaporate there is meant the time interval in seconds between the application of heating power and the onset of barium evaporation when there is the start of "flashing" due to onset of the exothermic reaction which is responsible for barium evaporation. The importance of knowing the start time is due to the fact that normally, as the total time of application of RF power to the coils is fixed, then a variation in start time to greater or lesser values than those recommended by the getter manufacturer can lead to, respectively a reduction in the barium yield or an overheating of the getter holder which may even melt. The disadvantages which can occur in the latter case are obvious but even in the former an insufficient yield of barium within the vacuum tube can result in a reduced life. With this in mind the getter manufacturer provides the user with graphs which show the getter "yield curves" which indicate, for various total times of R.F. generator functioning, the mass of barium evaporated as a function of start time or start of evaporation.
Hence it is important to measure, each time, the value of the start time which, up to now, has been measured by simple direct visual observation by an operator who is usually the person in charge of the R.F. generator. However, direct observation of the start time is extremely difficult, if not impossible, apart from, inevitable human errors, due to the fact that normally the kinescope glass is internally covered with an opaque layer based on graphite which is known as "Dag" by those skilled in the art. Even when a small "window" is left in the dag layer, corresponding to the getter position, the getter container, especially if using a ceramic support does not permit an adequate view within the tube.